Hypereosinophilic conditions have been studied extensively, and we have identified new markers that distinguish between a myeloproliferative and a lymphoproliferative form of the Hyperosinophilic Syndrome. Having made this distinction, we have instituted new therapies for the treatment of each of these forms. The interaction between helminth infection and allergic disease has been studied using epidemiological tools coupled with physiological measurements of allergic disease and immunological assessments. We have demonstrated that chronic helminth infection protects against allergic diseases.